Power supply device and method for limiting an output current of a power supply device

ABSTRACT

A switched-mode power supply system and method for converting an input voltage into an output voltage, including at least one switching stage controlled by a pulse-width modulation circuit in a clocked manner, and a control circuit that influences a pulse-width modulation circuit to vary the level of the output voltage, characterized by the provision of a current limiting circuit which, after a threshold voltage has been exceeded, limits the power supply output current first to an elevated maximum current for a first period of time, and thereafter to a regular maximum current. The control circuit is such that the first period of time for which the output current is limited to the elevated maximum current is dependent on the level of the output current.

REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 C.F.R. § 371of the PCT International Application No. PCT/EP2014/074839 filed Nov.18, 2014, which claims priority of the German application No. DE 10 2013113 648.6 filed Dec. 6, 2013.

BACKGROUND OF THE INVENTION Field of the Invention

A switched-mode power supply system and method for converting an inputvoltage into an output voltage, including at least one switching stagecontrolled by a pulse-width modulation circuit in a clocked manner, anda control circuit that influences a pulse-width modulation circuit tovary the level of the output voltage. A current limiting circuit isprovided which, after a threshold voltage has been exceeded, limits thepower supply output current first to an elevated maximum current for afirst period of time, and thereafter to a regular maximum current. Thelength of the first period of time is dependent on the level of theoutput current.

Description of Related Art

It is known in the prior art to provide power supply systems in which aninput voltage is converted by means of a switching stage into analternating current voltage the frequency of which usually lies in thekilohertz range. This input side higher frequency alternating-currentvoltage is transformed, by means of a transformer into an output-sidehigher frequency alternating—current voltage which is lower or higher,and is then rectified again. The power supply system in this case can beembodied as a direct current converter, also called a DC/DC converter,in which a direct-current voltage as input voltage is converted into adirect—current voltage as output voltage. The power supply system canalso be embodied as a so-called switched-mode power supply device, inwhich a mains alternating current voltage as input voltage is rectifiedand then converted to an output direct current voltage.

To stabilize the output direct-current voltage supplied by the powersupply system, a control circuit is provided which controls the outputvoltage to the most constant value possible, independently of aconnected load. This can be accomplished by changing the frequencyand/or the pulse width or duty factor of the clocked actuation of theswitching stage in a pulse-width modulation method (PWM). For thispurpose, the power supply system has a PWM switching stage, which isinfluenced by the control circuit.

In addition to maintaining the most constant output voltage possible,the control circuit also typically performs current limitation, in whichthe supplied output current is limited to a preset value by adjustingthe output voltage downwardly once the current value is reached, so thatthe preset maximum current value will not be exceeded.

Particularly in the case of power supply devices that supply highcurrents in the range of several to several tens of amperes to powersystems, such systems are provided with an overcurrent protector, forexample a safety fuse or e.g. a thermal and/or magnetic overcurrentprotective device, or a combination of such protective elements. Theovercurrent protective device prevents components and/or leads fromoverheating, particularly in the event of a fault. However, safety fusesand the aforementioned thermal or magnetic overcurrent protectivedevices are very slow-acting and also require tripping currents, whichcan amount to many times the nominal current for which the protector isdesigned. Power supply units that have a transformer which operatesdirectly at the power supply unit and at the unit frequency are usuallycapable of supplying a sufficiently high output current, many timesgreater than the nominal current, at least for a sufficient period oftime to ensure a reliable tripping of the protector.

With the power supply systems of the type described above, however, thecurrent limitation for protecting against thermal overloading of theswitched-mode power supply device is typically set to approximately 1.1to 1.5 times the nominal output current. Moreover, the currentlimitation of the switched-mode power supply device operates so quicklythat without additional measures, the protective elements connecteddownstream will be tripped too late or unreliably.

To ensure the reliable tripping of a safety fuse connected downstream ora thermal or magnetic overcurrent protective device connecteddownstream, for example, even in the case of a clocked power supplysystem, German patent No. DE 10 2005 031 833 A1 discloses a power supplysystem in which a very high output current, which is five to ten timesthe nominal current, for example, can be supplied for a predeterminedamount of time, and after the predetermined time has elapsed the currentlimitation is returned to a lower value of 1.1 to 1.5 times the nominalcurrent, for example. The higher maximum current is available during thepredetermined time for the reliable tripping of a safety fuse connecteddownstream or an overcurrent protective device connected downstream.Once the predetermined time has elapsed, only the maximum current thatis slightly higher than the nominal current is steadily supplied.Adjusting downwardly to the slightly elevated maximum current prevents athermal overload of the power supply system, in case a protective deviceis not provided downstream or has not been tripped. The latter can occurparticularly when, in the event of a fault, although the flowing currentis elevated, it is not high enough that the predetermined time issufficient to trip the protective device. This leads to an undesirableoperating state in which the elevated current continues to be suppliedto the system.

The present invention was developed to provide a power supply systemhaving a clocked switching stage, in which an overcurrent protectivedevice connected downstream will be tripped even in cases of anovercurrent in which the external overcurrent protective device is nottripped within the predetermined time.

SUMMARY OF THE INVENTION

Accordingly, a primary object of the present invention is to provide aswitched-mode power supply system and method for converting an inputvoltage into an output voltage, including at least one switching stagecontrolled by a pulse-width modulation circuit in a clocked manner, anda control circuit that influences a pulse-width modulation circuit tovary the level of the output voltage. A current limiting circuit isprovided which, after a threshold voltage has been exceeded, limits thepower supply output current first to an elevated maximum current for afirst period of time, and thereafter to a regular maximum current, thefirst period of time being dependent on the level of the output current.

According to another object the control circuit is configured such thatthe period of time during which the output current is limited to theelevated maximum current is dependent on the level of the outputcurrent. The elevated maximum current that flows to trip an overcurrentprotective device connected downstream is therefore not available for apredetermined—and thus possibly too short—period of time, but rather, isavailable, depending on the situation, for a period of time the durationof which is dependent on the level of current that is flowing in thespecific overcurrent case. The elevated maximum current is preferablybetween 5 and 10 times the nominal current of the power supply deviceand the regular maximum current is approximately between 1.1 times and1.5 times the nominal current.

According to a further object of the invention, in an advantageousembodiment of the power supply system, the longer the period of time,the smaller is the difference between the output current and thethreshold value that defines the overcurrent case during the statedperiod of time. For example, the threshold value can be set to the valueof the regular maximum current. In this embodiment, an output currentthat is not as high is able to flow for a longer time in the case of afault, until the power supply device adjusts downward from the elevatedmaximum current to the regular maximum current. The resulting timecharacteristic of the maximum current benefits the tripping of a safetyfuse or a thermal or magnetic overcurrent protective device, connecteddownstream of the power supply device. In the case of a very highcurrent, corresponding to 10 times the nominal current of the protectivedevice, for example, a shorter time is required for tripping theprotective device than with a lower current, corresponding to only 5times the nominal current of the protective device, for example. Sincethe thermal load of the power supply device is lower with a lowermaximum current than with a higher maximum current, the power supplydevice is able to offer this time characteristic without the risk of athermal overload of the power supply device while the elevated maximumcurrent is being supplied.

The time characteristic of the current limiting circuit also enablesloads to be powered with a high starting current, as is typically thecase with engine loads or capacitive loads. An engine starting currentof around 2.5 times the nominal current of the power supply device, forexample, can then be supplied for a correspondingly longer time ascompared with a short-circuit with an overcurrent of 10 times the levelof the nominal current, for example.

The power supply device can also be a DC/DC converter, or an AC/DCconverter, e.g. a switched-mode power supply device. The latter can beembodied for connection to single-phase or multi-phase networks, e.g.3-phase networks. The input voltages may range from 10 to 800 volts (V);nominal currents may range from several amperes to several tens ofamperes (A). The elevated maximum current may be supplied by means of anenergy store, for example a capacitor. However, in terms ofcurrent-carrying capacity, the power supply device may also be embodiedsuch that the capacity for supplying output current is provided directlyat the input of the power supply device.

In an advantageous embodiment of the power supply device, said devicehas a first comparison stage, which compares a voltage that isproportional to the output current with a first reference value, thefirst reference value corresponding to the level of the present maximumcurrent. The current is thereby limited to a maximum current. The powersupply device further preferably has an integrator circuit, via whichthe voltage, which is proportional to the output current, is suppliedfiltered to a second comparison stage, which compares the filteredvoltage with a second reference value that corresponds to the regularmaximum current, wherein an output of the second comparison stage iscoupled to the first comparison stage in such a way that it influencesthe level of the first reference value. The set maximum current istherefore lowered from the elevated to the regular maximum value once anintegration element of the integrator circuit has been “charged”. Theintegration results in a dynamic adjustment of the period of time duringwhich the elevated maximum current is supplied, based on the level ofcurrent flowing in the case of a fault.

The integrator circuit preferably has a capacitor as an integrationelement and at least one charging and/or discharging resistor in alow-pass assembly. In this case, a charging resistor having aseries-connected diode and a discharging resistor having an additionalseries-connected diode may be provided, the two series circuits beingconnected antiparallel to one another with respect to the diode or theadditional diode. The time characteristic of current limitation isadjusted by means of the charging and/or discharging resistor. A commoncharging and discharging resistor may be used for this purpose, oralternatively separate charging resistors and discharging resistors,with the direction of current in the resistor and therefore its function(charging/discharging) being determined in each case by a seriesconnection with the diode or the additional diode.

When, either by a tripping of the protective device connected downstreamor by a correction of the fault, the output current at the output of thepower supply device drops back below the regular maximum current, thecapacitor will be discharged via the discharging resistor. Only afterthe capacitor has been discharged to below the second reference value isthe elevated maximum current again available for the next fault. Thetime constant for the discharge can be selected by selecting thedischarging resistor such that an elevated maximum current can besupplied a second time by the power supply device only after the powersupply device has cooled again sufficiently. The time constant fordischarging the capacitor is therefore preferably adjusted to typicaltime constants for cooling the power supply device. In a furtherpreferred embodiment, a temperature-dependent resistor, in particular aPTC (positive temperature coefficient) resistor, is additionallyarranged in the series circuit of the discharging resistor and theadditional diode. The time constant for discharging the capacitor isthereby embodied as temperature dependent. When the temperature of thepower supply device or the component to which the temperature-dependentresistor is thermally contacted is higher, the time constant fordischarging the capacitor is lengthened, and therefore the regenerationtime during which no elevated maximum current is supplied is lengthenedwhen the temperature of the power supply device is elevated.

A method for limiting an output current of a power supply device,according to the invention, comprises the following steps: a currentlimitation for the output current is set to an elevated maximum current.When an output current that is above the threshold value is detected,the output current that lies above the threshold value but below thelevel of the elevated maximum current is supplied for a period of time,the length of which is dependent on the level of the detected outputcurrent. The current limitation of the output current is then adjustedto a regular maximum current that is lower than the elevated maximumcurrent. This method can be carried out particularly in theabove-described power supply device. The advantages described inconnection with the power supply device according to the inventionresult.

In an advantageous embodiment of the method, after a further period oftime, the current limitation is returned to the elevated maximumcurrent. The duration of this additional period of time can preferablybe dependent on the level of the detected output current during the timein which the current limitation is set to the regular maximum current.More preferably, the duration of the additional period of time can bedependent on a temperature measured in the power supply device.

In an advantageous embodiment of the method, the duration of the periodof time and optionally of the additional period of time is determined byan integration of a voltage that represents the output current. Theintegration results in a simple manner in a suitable time characteristicfor the dependence of the duration of the time period or of theadditional time period on the level of current that is flowing. Acurrent that does not remain constant for the duration of the timeperiod is also taken into consideration. The integration over thecurrent essentially also describes the thermal load to which the powersupply device is exposed during the period of overcurrent. The durationof the time period during which a current that is above the regularmaximum current can be supplied can therefore be adjusted to the settemperature of the power supply device or the temperature-criticalcomponents thereof during this time period.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent froma study of the following specification, when viewed in the light of theaccompanying drawing, in which:

FIG. 1 is a schematic circuit diagram of the switched-mode power supplysystem of the present invention;

FIG. 2 is a detailed circuit diagram of a first embodiment of thecontrol circuit of FIG. 1; and

FIG. 3 is a detailed circuit diagram of a modification of the controlcircuit of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring first more particularly to FIG. 1, a switched-mode powersupply system 1 is provided for converting an input voltage U_(E), inthis case an input alternating-current voltage, into an output voltageU_(A), in this case an output direct-current voltage.

Input voltage U_(E) is converted by means of a rectifier 2 into apulsing direct current voltage U₁, which is smoothed and/or filtered bymeans of a smoothing circuit 3. For this purpose, the smoothing circuit3 has a first smoothing capacitor C₁. Alternatively, an active powerfactor correction circuit (PFC=power factor correction) may also be usedas rectifier 2.

Direct current U₁ is fed in a clocked manner to a primary-side (I)winding of a transformer 5, via a switching stage 4 having a switchingelement 41. Switching stage 4 converts direct-current voltage U₁ into ahigher frequency alternating-current voltage U₂, the frequency of whichis significantly higher than the frequency of input alternating currentvoltage U_(E).

Alternating current voltage U₃ is converted by transformer 5 into asmaller (or in certain applications also greater) amount ofsecondary-side (II), higher frequency alternating current voltage U₃.The secondary-side, higher frequency alternating current voltage U₃ isthen rectified again in a secondary-side rectifier 6 into asecondary-side direct current voltage and is smoothed and/or filtered ina secondary-side smoothing circuit 7. For this purpose, thesecondary-side smoothing assembly 7 in this case has an additionalsmoothing capacitor C₂, by way of example. In principle, however, morecomplex circuits comprising a plurality of particularly discretecomponents (not shown) for the secondary-side smoothing assembly 7 arepreferred.

The output voltage of secondary-side smoothing circuit 7 is outputvoltage U_(A) of power supply device 1, which in this case is positiverelative to a reference potential GND.

In order for output voltage U_(A) to remain stable even with a changingload 10, a control circuit 8 is provided, which compares output voltageU_(A) with a reference voltage and, based on this comparison, influencesa pulse-width modulation (PWM) circuit 9. PWM circuit 9 controlsswitching stage 4 and modifies the clock parameters, in particular atiming ratio, but optionally also a timing frequency, of switching stage4, based on the data from control circuit 8, thereby influencing outputvoltage U_(A). Thus a control circuit is formed by which output voltageU_(A) is held at a desired, predetermined value.

In addition to this voltage control, a current control is also provided,in which process a current I_(A) flowing at the output of switched-modepower supply device 1 and delivered to load 10 is measured by a currentsensor 81, and the control circuit is configured to limit current I_(A)by lowering voltage U_(A) to a specifiable maximum current. Currentsensor 81 can be arranged as shown here, upstream of smoothing assembly7 or between said assembly and load 10, without any significant impacton the method of functioning. Details of the current limitation by meansof control circuit 8 will be described below in reference to FIGS. 2 and3.

A switched-mode power supply device 1 of this type frequently also has afilter (not shown), which is used for filtering input alternatingcurrent voltage U_(E) prior to rectification, in order to filter outharmonics, overvoltages and/or mains-borne interferences.

On the secondary side, transformer 5 may also have a plurality ofsecondary windings (not shown), which may be used to generatesecondary-side alternating current voltages U₃ of various levels. Inthis embodiment of switched-mode power supply device 1, a plurality ofrectifiers 6 and smoothing assemblies 7 would then be provided for eachof the different secondary-side alternating current voltages.

FIG. 2 shows a portion of control circuit 8 of switched-mode powersupply device 1 in greater detail. Illustrated here is the circuit forcurrent limitation within control circuit 8. The circuit for voltagecontrol in a switched-mode power supply device of this type is known inprinciple and therefore will not be described in greater detail in thisapplication.

Control circuit 8 has an evaluation amplifier 82 for the signal ofcurrent sensor 81. Evaluation amplifier 82 is connected to currentsensor 81 and has an output, where a voltage V₁ that is proportional tomeasured current I_(A) is output. A shunt resistor may act as currentsensor 81, in which case the voltage drop at the shunt resistor is ameasure of flowing current I_(A). Alternatively, a Hall sensor may beused for current measurement.

The voltage signal V₁ emitted by evaluation amplifier 82 is fed to afirst comparison stage 83, which has an operational amplifier 831 ascomparator. The signal V₁ which is proportional to output current I_(A)is fed to the non-inverting input of operational amplifier 831, whereasa first comparison voltage, which in the present case is generated froma first reference voltage V_(ref1) and a voltage divider, is fed to theinverting input. For this purpose, the inverting input of operationalamplifier 831 is connected via a resistor 832 to the first referencevoltage source (V_(ref1)), and is also connected via a further resistor833 to reference potential GND.

In addition, the inverting input of operational amplifier 831 is furtherconnected via an additional resistor 834 to an output of a secondcomparison stage 84. Initially, it is assumed that the output V₃ of thesecond comparison stage 84 is at a positive reference potential relativeto reference potential GND. In this case, the potential at the invertinginput of operational amplifier 831 results from first reference voltageV_(ref1) via resistor 832 or from the stated positive potential and theadditional resistor 834 on one hand, and the additional resistors 833 onthe other. The resistance values or potentials are selected in this casesuch that, at the output of operational amplifier 831—and therefore offirst comparison stage 83—a positive potential V₄ is established whenoutput current I_(A) of switched-mode power supply device 1 is greaterthan or equal to a specified elevated maximum current I′_(max). Theelevated maximum current I′_(max) is selected in this case as a highcurrent in the range of five to ten times the output nominal currentI_(nenn) of switched-mode power supply device 1.

When a positive potential is present at the output V₄ of firstoperational amplifier 831, a light-emitting diode of an optocoupler 86is switched on via a voltage drop resistor, not shown in greater detailin the figure, wherein optocoupler 86 influences the voltage controlbranch of control circuit 8, not shown here, causing a decrease inoutput voltage U_(A). Switched-mode power supply system 1 is therebylimited by first control circuit 83, via optocoupler 86, to elevatedmaximum current I′_(max).

Second comparison stage 84 likewise has an operational amplifier 841 ascomparator, to which a second reference value is fed at a non-invertinginput. This second reference value is formed in relation to referencepotential GND from second reference voltage V_(ref2) by means of avoltage divider comprising resistors 842 and 843. The value that will becompared with this reference value is fed to the inverting input ofoperational amplifier 841.

Second comparison stage 84 supplies at its output the abovementionedpositive voltage value V₃ when the input voltage V₂ of second comparisonstage 84 is below the second reference value V_(ref2). If the voltage V₂at the input of second comparison stage 84 exceeds the second referencevalue V_(ref2), a negative potential or, depending on the supply voltageof operational amplifier 841, reference potential GND, will be output atthe output of second comparison stage 84. Thus, the first referencevalue present at the inverting input of operational amplifier 831 offirst comparison stage 83, via resistor 834, is dependent upon thepotential V₃ at the output of second comparison stage 84.

More specifically, when the second reference value V_(ref2) is exceededat the input V₂ of second comparison stage 84, the current limitation isdecreased from the specified elevated maximum current I′_(max) to aregular maximum current I_(max). Thus the interconnection of the twocomparison stages 83, 84 results in a limitation of output current I_(A)either to elevated maximum current I′_(max) or to regular maximumcurrent I_(max). Regular maximum current I_(max) can be determined bythe selection of the resistance values of resistors 832, 833 and 834 andby the potentials V₃ present at the output of operational amplifier 841of second comparison stage 84. Regular maximum current I_(max) isadvantageously set to a value within the range of 1.1 to approximately1.5 times the output nominal value I_(nom) of switched-mode power supplydevice 1.

The switchover between the two maximum currents I′_(max), I_(max) isimplemented by means of an integrator circuit 85, which is connectedupstream of the input of second comparison stage 84. On the input side,integrator circuit 85 is connected to the output of evaluation amplifier82, and is thus supplied the voltage V₁ that is proportional to outputcurrent I_(A) in the same manner as the input of first comparison stage83.

Integrator circuit 85 has a charging resistor 851 which is connected tothe input, and which is connected via a diode 852 to a capacitor 855that is connected to reference potential GND. Parallel to chargingresistor 851 and diode 852, a branch having a series circuit comprisinga discharging resistor 853 and an additional diode 854 is connected. Theadditional diode 854 is arranged in the opposite direction from diode852. Integrator circuit 85 is thus configured as a low-pass circuit,however capacitor 855, and therefore the output of integrator circuit85, is charged to a potential which is positive in relation to referencepotential GND via charging resistor 851, and capacitor 855 is dischargedvia discharging resistor 853. By properly selecting the resistancevalues for resistors 851 and 853, different time constants for thecharging and discharging of capacitor 855 may be selected. In principle,a circuit comprising only one common charging and discharging resistoris also conceivable, which would result in equal time constants for thecharging and discharging of the resistor. In addition, parallel tocapacitor 855 an additional discharging resistor 856 is arranged, whichhas a high resistance value relative to charging resistor 851 anddischarging resistor 853, and which serves substantially to dischargecapacitor 855 even when switched-mode power supply device 1 is switchedoff.

During operation of switched-mode power supply device 1, the output V₂of integrator circuit 85 substantially follows the voltage V₁ suppliedat the output of evaluation amplifier 82 and reflects output currentI_(A). In the event of a fault, for example a short-circuit at theoutput of switched-mode power supply device 1, output current I_(A) willincrease from a value below output nominal current I_(nom) to a maximumof the elevated maximum value I′_(max), which is adjusted at firstcomparison stage 83, since the voltage V₃ at the output of integratorcircuit 85 is at first lower than the second reference voltage V_(ref2).

This voltage V₂ at the output of integrator circuit 85 follows thevoltage jump at the output of evaluation amplifier 82 only slowly,namely with the time constant determined by the resistance value ofcharging resistor 851 and the capacitance of capacitor 855. As long asthe output V₂ of integrator circuit 85 remains below the secondreference value, the current limitation will remain at elevated maximumvalue I′_(max). Once it exceeds the second reference value, the currentlimitation is lowered to regular maximum current I_(max). Theswitched-mode power supply device then optionally remains steadily inthis state.

In the event of a fault, if output current I_(A) increases to a currentvalue that is above regular maximum current I_(max) but does not reachelevated maximum current I′_(max), it will take correspondingly longerbefore the voltage V₂ at the output of integrator circuit 85 exceeds thesecond reference value. Therefore, in the event of a fault, an outputcurrent I_(A) that is not as high can continue to flow for a longer timebefore switched-mode power supply device 1 is adjusted downward fromelevated maximum current I′_(max) to regular maximum current I_(max).This results in a time dependence of the maximum supplied outputcurrent, which is dependent on output current level I_(A).

This characteristic has an advantageous effect on the tripping of asafety fuse or of a thermal or magnetic overcurrent protective device,connected downstream of switched-mode power supply device 1. In the caseof a very high current, corresponding to 10 times the nominal current ofthe protective device, for example, less time is required for trippingthe protective device than with a lower current that is only 5 times thenominal current of the protective device, for example. Since the thermalload is lower with a lower maximum current for switched-mode powersupply system 1 than with a higher maximum current, switched-mode powersupply device 1 is able to supply this time characteristic without riskof a thermal overload of switched-mode power supply device 1 while theelevated maximum current is being supplied.

The time characteristic of the current limiting circuit also enables thepowering of loads with a high starting current, as is typically the casewith engine loads or capacitive loads. An engine starting current ofaround 2.5 times the nominal current of the power supply device, forexample, can then be supplied for a correspondingly longer time ascompared with a short-circuit with an overcurrent of 10 times the levelof the nominal current.

When output current I_(A) at the output of switched-mode power supplydevice 1 drops back below regular maximum current I_(max), either due toa tripping of the protective device downstream or by a correction of thefault, capacitor 855 is discharged via additional diode 854 anddischarging resistor 853. When capacitor 855 is discharged below thesecond reference value, the next time a fault occurs the elevatedmaximum current I′_(max) will again be initially available. The timeconstant for discharge can be selected by selecting resistor 853 suchthat switched-mode power supply device 1 is able to supply an increasedmaximum current again only after switched-mode power supply device 1 hascooled sufficiently. The time constant for discharging capacitor 855 istherefore preferably adapted to typical time constants for coolingswitched-mode power supply device 1.

In the same manner as FIG. 2, FIG. 3 shows a further embodiment exampleof a portion of control circuit 8 of a switched-mode power supply device1 according to the invention. Similar or similarly functioning elementsare identified in this embodiment example by the same reference signs.In terms of its basic structure, the circuit shown in FIG. 2 correspondsto the circuit shown in FIG. 2, the description of which is providedherewith.

In contrast to the embodiment example of FIG. 2, an additionaldischarging resistor 857 is provided in the discharge branch ofintegrator circuit 85, within the series circuit of additional diode 854and discharging resistor 853, said additional discharging resistorhaving a temperature-dependent resistance value, and being particularlyformed by a PTC (positive temperature coefficient) resistor. Theadditional discharging resistor 857 changes its resistance value basedon the temperature of switched-mode power supply device 1, in particularbased on the temperature of a component, the temperature of whichchanges dramatically dependent on the load, for example the temperatureof the output-side rectifier stage 6. The time constant for dischargingthe capacitor 855 is therefore temperature dependent. When thetemperature of switched-mode power supply device 1 or the component withwhich the additional discharging resistor 857 is in thermal contact ishigher, the time constant for discharging the capacitor 855 islengthened. Therefore, when switched-mode power supply device 1 is at anelevated temperature, the regeneration time available to switched-modepower supply device 1, during which switched-mode power supply device 1cannot supply an elevated maximum current I′_(max), is lengthened.

While in accordance with the provisions of the Patent Statutes thepreferred forms and embodiments of the invention have been illustratedand described, it will be apparent to those skilled in the art thatchanges may be made without deviating from the invention describedabove.

What is claimed is:
 1. A switched-mode power supply for converting aninput voltage to an output voltage, comprising: (a) a transformer havingprimary and secondary windings: (b) at least one switching stage circuitwhich supplies said input voltage to said transformer primary winding;thereby causing said secondary winding to produce an output current andan output voltage, said switching stage circuit including a switchingelement: (c) a pulse-width modulation circuit connected with saidswitching stage switching element to control in a clocked manner theoperation of said switching stage circuit: and (d) a control circuitwhich controls the operation of said pulse-width modulation circuit tovary the level of the output voltage, said control circuit including acurrent limiting circuit for establishing a current threshold level andoperable when said current threshold level has been exceeded by avoltage signal that is a function of the output current to limit for afirst period of time the output current to an elevated maximum current,and thereafter to limit the output current to a regular maximum current,said first period of time being dependent on the level of said outputcurrent, said current limiting circuit including (1) a first comparisoncircuit for comparing a first voltage signal with a first referencevoltage, thereby to produce an output voltage signal that controls theoperation of said pulse-width modulation circuit, and thereby controlthe level of said output voltage; (2) an integrator circuit forfiltering said first voltage signal to produce a second voltage signal;and (3) a second comparison circuit for comparing said second voltagesignal with a second reference voltage corresponding with said regularmaximum current, thereby to produce a third voltage signal that issupplied as an input to said first comparison circuit to influence theoperation of said first comparison circuit.
 2. A switched-mode powersupply as defined in claim 1, wherein the longer said first period oftime exists, the smaller is the difference between said first voltagesignal and said first threshold value during said first period of time.3. A switched-mode power supply as defined in claim 1, wherein saidintegrator circuit comprises a low-pass filter including at least onecharging/discharging resistor and a capacitor.
 4. A switched-mode powersupply as defined in claim 1, wherein said integrator circuit includes afirst branch including a charging resistor connected in series with afirst diode having a first polarity, and a second branch connected inparallel with said first branch, said second branch including adischarging resistor connected in series with a second diode of theopposite polarity.
 5. A switched-mode power supply arrangement asdefined in claim 4, and further including a temperature-dependentresistor connected in series in said second branch.
 6. A switched-modepower supply as defined in claim 1, wherein said input voltage has arange of from between about 10 volts to about 800 volts.
 7. Aswitched-mode power supply as defined in claim 1, wherein said inputvoltage has a range of from about 15 volts to about 265 volts.
 8. Aswitched-node power supply as defined in claim 1, wherein said input,voltage is an alternating-current voltage, and said output voltage is adirect-current voltage.
 9. A switched-mode power supply as defined inclaim 1, wherein said output current normally has a nominal currentvalue, and further wherein said elevated maximum current is between 5 to10 times said nominal current value, and said regular maximum current isbetween 1.1 and 1.5 times said nominal current.
 10. A switched-modepower supply for converting an input voltage to an output voltage,comprising: (a) a transformer having primary and secondary windings; (b)at least one switching stage circuit for supplying said input voltage tosaid transformer primary winding, thereby causing said transformersecondary winding to produce an output current and an output voltage,said switching stage circuit containing a switching element; (c) apulse-width modulation circuit connected with said switching stageswitching element for controlling in a clocked manner the operation ofsaid switching stage circuit; and (d) a control circuit controlling theoperation of said pulse-width modulation circuit to vary the level ofthe output voltage, said control circuit including: (1) a firstcomparison stage including: (a) a first operational amplifier having anoutput terminal connected with said pulse-width modulation circuit, anda pair of input terminals; and (b) a first reference voltage connectedwith a first one of said first operational amplifier input terminals;(2) a sensor which applies a first voltage signal that is a function ofthe output current to a second input terminal of said first operationalamplifier, whereby upon the occurrence of an overload, an elevatedmaximum current is permitted; (3) an integrator circuit for generatingfrom said first voltage signal a time-integrated second voltage signal;(4) a second comparison stage including: (a) a second operationalamplifier having an output terminal connected with said firstoperational amplifier first input terminal, and a pair of inputterminals; and (b) a second reference voltage connected with a firstinput terminal of said second operational amplifier, the second input ofsaid second operational amplifier being connected with said integratorcircuit to receive said time-integrated second voltage signal, therebyto produce a third voltage signal at the output terminal of said secondoperational amplifier that influences the operation of said firstoperational amplifier; (5) said control circuit being operable when thethreshold level established by said second reference voltage has beenexceeded to limit for a first period of time the output current at theelevated maximum current, and thereafter to limit the output current toa regular maximum current, said first period of time being dependent onthe level of said output current.